White space usage for wireleess local area network devices

ABSTRACT

A method and apparatus are described including defining a neighbor set for each access point, selecting a first clock in a first access point, the selected clock having a highest accuracy as a grand master clock, advising neighboring access points to synchronize with the selected grand master clock and transmitting a message to schedule a quiet period based on the grand master clock. Also described are a method and apparatus including receiving a beacon message, inspecting clock descriptors in the beacon message, selecting a best master clock responsive to the inspection and transmitting a message to schedule a quiet period based on the selected best master clock.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for wirelesslocal area network (WLAN) devices to operate in unoccupied portions ofthe digital broadcast television spectrum.

BACKGROUND OF THE INVENTION

In multicast and broadcast applications, data are transmitted from aserver to multiple receivers over wired and/or wireless networks. Amulticast system as used herein is a system in which a server transmitsthe same data to multiple receivers simultaneously, where the multiplereceivers form a subset of all the receivers up to and including all ofthe receivers. A broadcast system is a system in which a servertransmits the same data to all of the receivers simultaneously. That is,a multicast system by definition can include a broadcast system.

The use of unoccupied digital TV spectrum by unlicensed radiotransmitters has not before been addressed since the transition of TVbroadcasting from analog to digital was only recently completed.

The FCC has issued a ruling that allows the usage of TV white space,i.e., TV channel 2-51 that are not being used by incumbent users, ifcertain requirements are met.

Specifically, WLAN devices need to sense the incumbent usage, needmechanisms for coexistence and resource sharing, and need to adapt theirtransmission characteristics to fit into one or more digital TV (DTV)channels.

SUMMARY OF THE INVENTION

When multiple TV white space (TVWS) devices want to access the same TVchannel, a resource sharing mechanism is required. This is known as acoexistence problem of heterogeneous systems in TVWS.

A method and apparatus are described including defining a neighbor setfor each access point, selecting a first clock in a first access point,the selected clock having a highest accuracy as a grand master clock,advising neighboring access points to synchronize with the selectedgrand master clock and transmitting a message to schedule a quiet periodbased on the grand master clock. In case multiple clocks have the samehighest accuracy then the clock having the lowest media access control(MAC) address is selected from among the multiple clocks. This mightoccur if more than one AP can receive a GPS or atomic clock. Alsodescribed are a method and apparatus including receiving a beaconmessage, inspecting clock descriptors in the beacon message, selecting abest master clock responsive to the inspection and transmitting amessage to schedule a quiet period based on the selected best masterclock.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Thedrawings include the following figures briefly described below:

FIG. 1 shows the three contiguous DTV channels for channel bonding.

FIG. 2 shows the reduction of the WLAN signaling bandwidth by removingthe edge carriers.

FIG. 3 is an example of an orthogonal frequency division multiplexing(OFDM) symbol for use in modified WLAN transmissions.

FIG. 4 is a schematic diagram of multiple WLAN networks.

FIG. 5 is a schematic diagram of synchronization of multiple accesspoints (APs).

FIG. 6 shows an exemplary modified beacon frame.

FIG. 7 is a flowchart of an exemplary embodiment of the global clockselection method in accordance with the principles of the presentinvention.

FIG. 8 is a flowchart of an exemplary embodiment of the local clockselection method in accordance with the principles of the presentinvention.

FIG. 9 is a block diagram of a device operating in accordance with theprinciples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In addressing the issue of the use of TV white space (TVWS) by WLANdevices, there are two main areas to consider. The first is modificationof WLAN transmission characteristics and the second is spectrum sensingand quiet period (QP) synchronization.

It is known that IEEE 802.11 a/g transmission requires a 20 MHz channelbandwidth, while IEEE 802.11n transmission requires 20 MHz or 40 MHzchannel bandwidth depending on the selection of the modulation codingscheme (MCS). However, in the DTV spectrum, the bandwidth of a channelis 6 MHz in US and 7 or 8 MHz in Europe or Asia. For exemplary purposes,a 6 MHz DTV channel used herein in all examples and illustrations. Thesame concept is equally applicable to 7 or 8 MHz DTV channels. Inaddition, there are a variety of WLAN devices such as devices that arecompatible with IEEE 802.11 a/g/n standards. For illustrative purposes,IEEE 802.11a devices are used for illustrations.

To resolve the channel mismatch between the 20 MHz (or 40 MHz) bandwidthused by most WLAN devices and the 6 MHz bandwidth for DTV channels, twomethods are described and analyzed below. The first option is channelbonding and the second option is channel scaling.

FIG. 1 shows the bonding of three contiguous white space DTV channels toform an 18 MHz channel. In IEEE 802.11a, carrier indices from −26 to 26are used for data transmissions and pilot carrier. The total occupiedbandwidth for IEEE 802.11a device is 20 MHz. In the three-channelbonding case, since the bandwidth has been reduced by (20−18)/20=10%,the total number of usable carriers is correspondingly reduced to theset with indices from −24 to 24, as shown in FIG. 2. In this way, thenumber of carriers is reduced by 2/26≈8%.

In the modified IEEE 802.11a transmitter, the formation of the modifiedpreamble is same as a regular preamble except the carriers −26, −25, 25,26 are removed. The formation of the data symbol should take intoconsideration the reduced number of carriers. The corresponding datarate for each modulation coding scheme should be modified accordingly aswell. Thus, the computation of the transmission time needs to bemodified.

The advantage of this method is the modification of the IEEE 802.11a PHYand MAC layer are minimal. The disadvantage is that three consecutive(contiguous) white space DTV channels are difficult to find in mostregions of the USA. In addition, since each modified WLAN transmissionrequires three channels, it is also difficult to facilitate resourcesharing and coexistence among WLAN transmissions due to the limitednumber of 18 MHz channels that can be formed.

In an alternative embodiment of the present invention, the total numberof usable carriers in the set can be further reduced by removingcarriers −24 and 24 such that the number of carrier is reduced by3/26=12%.

The second option is channel scaling. The sample rate for an IEEE802.11a device is 20 Mega-samples per second and the correspondingoccupied channel band width is also 20 MHz. Thus, the sample rate can bemodified in order to change the occupied bandwidth of the modified WLANtransmissions.

For example, the sample rate can be changed to 6 Mega-samples persecond, yielding an occupied channel bandwidth of 6 MHz. Thus, the WLANtransmissions will fit into a single DTV white space channel. Similarly,if there are two consecutive DTV white space channels available, thesample rate can be modified to 12 Mega-samples per second. For threeconsecutive white space channels, the sample rate will be 18 Meg-samplesper second. The advantage of this method is that the modification of aWLAN transceiver is minimal, i.e., the sample clock. In general, thesample clock or other system clocks are derived from phase locked loop(PLL) circuitry. The parameters of the PLL can be changed to modify thesample clock and other corresponding clock signals. Due to therequirement that any use of the TV white space cannot interfere withincumbent users, the transmission signal from WLAN devices in TV whitespace should be filtered using pulse shaping filters. Thus, the pulseshaping filter needs to be modified based on the available channelbandwidth and the sample rate.

Note that the duration of an OFDM symbol in the modified WLANtransmissions is also changed. Based on using a 64 point inverse fastFourier transform (IFFT), a symbol has 64 sample points. For example, asshown in FIG. 3, if the sample rate is 6 Mega-samples per second, thenthe duration of an OFDM symbol is

${\frac{1}{6} \times 64\mspace{14mu} {µs}} = {\frac{32}{3}\mspace{14mu} {µs}}$

Since the cyclic prefix is 1/4 of the length of a (OFDM) symbol, 1/4 isadded to 1 to yield a total symbol length of 5/4 of an OFDM symbolwithout prefix. After adding the cyclic pre-fix, the total length of anOFDM symbol thus becomes

${\frac{32}{3} \times \frac{5}{4}\mspace{14mu} {µs}} = {\frac{40}{3}\mspace{14mu} {µs}}$

The length of the preamble is also changed accordingly. Thus, thecomputation of transmission time is also changed.

The second issue is spectrum sensing and quiet period (QP)synchronization. Based on the FCC ruling, TV white space devices shouldaccess a geo-location database to identify the DTV channels in a region(geo-location) that are not occupied by TV stations. Since wirelessmicrophones are also licensed secondary users in TV channels, WLANdevices in TV white space should be able to identify the TV channelsused by wireless microphone transmissions. If wireless microphone usagein a given geo-location is also registered in the database, such aswireless microphones used in sporting or show events, spectrum sensingmay not be needed. However, the usage of wireless microphones for newsgathering is usually unpredictable and hence, cannot be registered inthe database beforehand. To identify those unregistered wirelessmicrophone usages, spectrum sensing is required.

Since the sensing threshold for wireless microphones is −114 dbm, WLANdevices should be quiet when sensing wireless microphone signals in agiven DTV channel. Thus, synchronization of WLAN transmissions isrequired. In this way, the quiet period for sensing can also besynchronized. In the following, methods for synchronization of WLANdevices in TV white space and methods for scheduling the quiet periodare described.

In terms of synchronization of WLAN devices in TV white space, considera WLAN network in infrastructure mode in which multiple stationscommunicate with an access point (AP). Assume multiple WLAN networksoperate in a given DTV white space channel, as shown in FIG. 4.

In a WLAN network, stations are synchronized with the AP through a timersynchronization function (TSF). The TSF maintains a 64-bit timer runningat 1 MHz. The TSF timer in the AP is reset to zero upon initializationand is then incremented by the 1 MHz clock of the AP. The AP sends abeacon frame periodically. At each beacon, the current value of thetimer is inserted in the beacon frame. A station receiving the beaconframe updates its TSF timer with the value of the timer it receives fromthe AP in the beacon frame, modified by any processing time required toperform the update operation. Thus, the timer values of all of thestations in the network receiving the beacon are synchronized to that ofthe AP.

However, synchronization within a network is not enough. Synchronizationamong multiple networks operating in the same white space channel isrequired. To achieve this goal, only the APs in these networks need tobe synchronized.

If all the APs are equipped with a Global Positioning System (GPS)receiver, which can receive a GPS satellite signal reliably, the APs canbe synchronized to the received satellite clock signal that is derivedfrom precise atomic clocks. However, APs may not be equipped with GPSreceivers or may not receive the GPS satellite signals reliablyespecially if they are located in an indoor environment. Alternativesynchronization methods for the TSF timers may be required.

One option is to synchronize to the fastest (or slowest) clocks in theneighborhood. An access point (AP) receives beacons from APs in theneighborhood. If the time stamp in the beacon is larger than (or lessthan) the value in the local TSF timer, the local TSF timer value isreplaced by the time stamp in the received beacon frame, modified by anyprocessing time required to perform the update operation. In this way,all the TSF timers of the APs in a region will be synchronized to thefastest clock (or the slowest clock) in the region. However, the fastest(or the slowest) clock may not be the best clock in terms of accuracyand stability. Note that the beacon signals can be sent in an operatingchannel or in a common control channel.

An alternative option is synchronization through backhaul links andover-the-air communications. Network time protocol (NTP) is usually usedto synchronize computer clocks in a computer network such as theInternet. However, NTP suffers from coarse accuracy, on the order oftens of milliseconds. The synchronization accuracy in WLAN networkshould be on the order of micro-seconds. Thus, NTP is not suitable forsynchronization of network clocks for the use of TVWS by WLAN devices.

IEEE 1588 is a protocol for precision clock synchronization, which canprovide accuracy on the order of micro-seconds. However, the protocolrequires the devices to be synchronized should be in a subnet or in afew local subnets that are connected by switches equipped with goodboundary clocks. Since APs in WLAN networks may be in different subnetsand the switches may not have good boundary clocks. IEEE 1588 may not bedirectly applied for synchronization of network clocks for the use ofTVWS by WLAN devices.

The IEEE 1588 protocol can however, be modified and applied forsynchronization of network clocks for the use of TVWS by WLAN devices.Assume all the APs in a region have backhaul links that connect to aspectrum server in the Internet. During the installation phase or thestart-up phase, the location, transmission power and the clockdescriptor of the APs are registered in the spectrum server. The clockdescriptor contains clock characteristics such as the clock accuracy,stability and the source of the clock. In addition, APs can also reportthe set of APs from which it can receive and decode data and clocksignals correctly to the spectrum server.

Since an AP can only hear beacon frames from APs in its neighborhood andthe spectrum server knows the topology and clock characteristics of allthe APs, the spectrum server can select the clock from one of the APs inthe neighborhood as a master clock and inform the corresponding APs tosynchronize with the selected master clock by replacing the local TSFtimer value with the time stamp in the beacon frame of the AP with themaster clock, modified by the processing time used for this operation.To achieve higher (better) accuracy, messages similar to IEEE 1588messages such as Sync message, Follow_Up message, Delay_Req message andDelay_Resp message can be used to compute the propagation delay, whichis then used to adjust local TSF timer value.

In one embodiment of the present invention, for a given AP, the spectrumserver can select the master clock in the AP's neighborhood based onbest master clock algorithms defined in IEEE 1588 and inform the APsthrough a backhaul link. Since the best master clock algorithm uses theclock descriptors of APs in the neighborhood, it is a local selectionmethod.

In an alternative embodiment of the present invention, a spectrum servercan do better in terms of clock selection. Since the spectrum server hasglobal knowledge of the clock characteristics of APs in the region, itcan select the best clock as the grand master clock in this region. Forexample, the best clock can be extracted from received GPS signals orfrom an atomic clock. In case multiple clocks have the same highestaccuracy then the clock having the lowest media access control (MAC)address is selected from among the multiple clocks. This might occur ifmore than one AP can receive a GPS or atomic clock. The spectrum serverinforms the neighbor APs that can hear the beacon of the grand master APto synchronize their clocks with the grand master clock using the beaconsignal of the grand master AP with any necessary adjustment forprocessing. Since the neighboring APs have synchronized with the grandmaster clock, they can be selected as master clock for their ownneighboring APs. In this way, all the clocks in this region aresynchronized to the grand master clock. This global selection methodshould perform better than the local selection method described above.

FIG. 5 is used as an example. The method described below works with boththe global selection method and the local selection method describedabove. Define the neighbor set of APx as the set of APs such that theAPx can receive and decode the beacon frames sent by the APs in the set.Denote the neighbor set of APx as, N(x). Thus, FIG. 5 has the followingneighbor sets:

N(1)={2,3,4}

N(2)={1},

N(3)={1},

N(4)={1,5,6}

N(5)={4,6}

N(6)={4,5}

Now assume the clock in AP1 has the highest accuracy and stability. AP1may have the highest accuracy if AP1 can and has received a GPS oratomic clock and synchronized its clock to the GPS or atomic clock orbecause it has the best local master clock. AP1 is chosen as the grandmaster clock. The spectrum server informs the neighbor APs that can hearthe beacon of the grand master AP to synchronize their clocks with thegrand master clock using the beacon signal of the grand master AP withany necessary adjustment for processing. Since the neighboring APs havesynchronized with the grand master clock, they can be selected as masterclock for their own neighboring APs. In this way, all the clocks in thisregion are synchronized to the grand master clock. Thus, the APs inAp1's neighborhood, i.e., AP2, AP3 and AP4, will synchronize theirclocks to the clock of AP1. Since AP4, AP5 and AP6 can hear each otherand AP4 has been synchronized to the grand master clock, AP4 will bechosen as the master clock for AP5 and AP6.

In the situation where there is no central spectrum server, the AP hasto select the clock by itself. In this case, each AP will send its clockdescriptor together with the time stamp in its beacon signal. Note thatthe clock descriptor is a new field added to a beacon frame. Each AP canrun the best master clock algorithm defined in IEEE 1588 to select themaster clock based on the beacon frames it can hear in its neighborhood.

An additional field can be added to the beacon frame, i.e., its masterclock descriptor, to describe the master clock to which it issynchronized. The master clock descriptor is defined as the clockdescriptor of itself if it is not synchronized to any other clock or thedescriptor of its master clock. In this way, the master clock field cantrack back to the grand master clock. Now the clock selection method canbe modified as follows: For a given AP, the AP first runs “the bestmaster clock algorithm” using the master clock descriptor. If there is atie, it then runs “best master clock algorithm” using the clockdescriptor of the clock of the tied APs and selects the best one as theits master clock.

In terms of scheduling quiet periods in order to sense the channels,synchronization must be accomplished first. If synchronization has notbeen accomplished then it will not be possible to discern if anymicrophones are using a given DTV channel.

After all the APs in a region (geo-location) are synchronized, thespectrum server can schedule the quiet period. For example, the spectrumserver can send a message to all the APs through a backhaul link to askthe APs to be quiet when their TSF counter is in the range of [x,y],where y-x determines the length of the quiet period. This is calledcentralized quiet period scheduling.

If there is no spectrum server, a distributed quiet period schedulingmethod is needed. In this case, the APs with a master clock or the grandmaster clock will send a quiet period scheduling message in the beaconframe or send the QP scheduling message (signal) separately to informits neighboring APs to schedule a QP. The modified beacon frame formatis shown in FIG. 6.

FIG. 7 is a flowchart of an exemplary embodiment of the global clockselection method in accordance with the principles of the presentinvention. The depicted global clock selection method operates in aspectrum server. At 705, the spectrum server defines a neighbor set foreach AP such that neighbors can receive and decode beacon messages(frames) sent by the APs in the neighbor set. At 710, the spectrumserver selects the clock with the highest accuracy as the grand masterclock based on knowledge of characteristics of clocks in the region(geo-location). In case multiple clocks have the same highest accuracythen the clock having the lowest media access control (MAC) address isselected from among the multiple clocks. This might occur if more thanone AP can receive a GPS or atomic clock. At 715, the spectrum serveradvises neighbor APs to synchronize to the selected grand master clock.At 720, the spectrum server selects an AP as a master clock from amongthose APs that can receive and decode beacon messages (frames) sent bythe APs in the neighbor set to act as master clock for APs that cannotreceive and decode beacon messages (frames) sent by the AP which has thegrand master clock but can receive and decode beacon messages (frames)sent by the AP acting as the master clock. At 725, the spectrum serversends out a message schedules a quiet period to determine if there areany microphones operating in a given DTV channel. All time stamps in allbeacon messages (frames) have to be modified by the recipient to accountfor processing and transmission time.

FIG. 8 is a flowchart of an exemplary embodiment of the local clockselection method in accordance with the principles of the presentinvention. The depicted local clock selection method operates in eachAP. At 805, the AP receives a beacon frame. At 810, the AP inspects theclock descriptors in the received beacon frame. At 815, the AP executebest master clock method in IEEE 1588 to select the best master clockbased on the beacon frames heard. At 820, the AP sends out a messageschedules a quiet period to determine if there are any microphonesoperating in a given DTV channel. All time stamps in all beacon messages(frames) have to be modified by the recipient to account for processingand transmission time.

FIG. 9 is a block diagram of a device operating in accordance with theprinciples of the present invention. The device depicted can be either aspectrum server or an AP. The transceiver actually transmits andreceives data and any control signals and the control logic performs allother functions.

Specifically, when operating as a spectrum server, the control logicmodule of device of FIG. 9 includes means for defining a neighbor setfor each access point, means for selecting a first clock in a firstaccess point, the selected clock having a highest accuracy as a grandmaster clock and means for selecting a second clock in a second accesspoint as a master clock, the second access point being able to receiveand decode beacon messages transmitted by the first access pointsynchronized with the grand master clock. In case multiple clocks havethe same highest accuracy then the clock having the lowest media accesscontrol (MAC) address is selected from among the multiple clocks. Thismight occur if more than one AP can receive a GPS or atomic clock. Thetransceiver modules of the device of FIG. 9 includes means for advisingneighboring access points to synchronize with the selected grand masterclock and means for transmitting a message to schedule a quiet periodbased on the grand master clock.

Specifically, when operating as an access point (AP), the transceivermodule of the device of FIG. 9 includes means for receiving a beaconmessage. The control logic module of the device of FIG. 9 includes meansfor inspecting clock descriptors in the beacon message, means forselecting a best master clock responsive to the inspection and means fortransmitting a message to schedule a quiet period based on the selectedbest master clock.

To ensure that an AP is aware of its status as having the grand masterclock or a master clock, its neighboring APs will send master clock andgrand master clock selection messages to the APs with a master clock orthe grand master clock. If APx with master clock status is synchronizedto another AP, e.g., APy, then APx will use the same quiet periodscheduling message in the beacon frame of APy as in its own beaconframe. In this way, the quiet period scheduling message of the AP withthe grand master clock or a master clock can propagate the QP schedulingmessage (signal) through the networks.

It is to be understood that the present invention may be implemented invarious forms of hardware, software, firmware, special purposeprocessors, or a combination thereof. Preferably, the present inventionis implemented as a combination of hardware and software. Moreover, thesoftware is preferably implemented as an application program tangiblyembodied on a program storage device. The application program may beuploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof), which is executed viathe operating system. In addition, various other peripheral devices maybe connected to the computer platform such as an additional data storagedevice and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying figuresare preferably implemented in software, the actual connections betweenthe system components (or the process steps) may differ depending uponthe manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the related art will be ableto contemplate these and similar implementations or configurations ofthe present invention.

1. A method, said method comprising: receiving a beacon message, saidbeacon message having a clock descriptor field, a master clockdescriptor field and a quiet period scheduling field; inspecting saidclock descriptor field and said master clock descriptor field in saidbeacon message; selecting a best master clock responsive to saidinspection; and transmitting a message to schedule a quiet period basedon said selected best master clock, wherein said best master clock isone of most stable, most accurate, fastest and slowest.
 2. The methodaccording to claim 1, wherein said method operates in an access point.3. The method according to claim 1, wherein said selection is performedby executing IEEE 1588 best master clock method.
 4. The method accordingto claim 1, wherein recipients of said transmitted message modify a timestamp in said messages responsive to time to receive and process saidmessages.
 5. An apparatus comprising: means for receiving a beaconmessage, said beacon message having a clock descriptor field, a masterclock descriptor field and a quiet period scheduling field; means forinspecting said clock descriptors field and said master clock descriptorfield in said beacon message; means for selecting a best master clockresponsive to said inspection; and means for transmitting a message toschedule a quiet period based on said selected best master clock,wherein said best master clock is one of most stable, most accurate,fastest and slowest.
 6. The apparatus according to claim 5, wherein saidapparatus is an access point.
 7. The apparatus according to claim 5,wherein said means for selection is performed by executing IEEE 1588best master clock method.
 8. The apparatus according to claim 5, whereinrecipients of said transmitted message modify a time stamp in saidmessages responsive to time to receive and process said messages.
 9. Anapparatus comprising: a transceiver, said transceiver receiving a beaconmessage, said beacon message having a clock descriptor field, a masterclock descriptor field and a quiet period scheduling field; controllogic, said control logic inspecting said clock descriptor field andsaid master clock descriptor field in said beacon message, said controllogic in communication with said transceiver; said control logicselecting a best master clock responsive to said inspection; and saidtransceiver transmitting a message to schedule a quiet period based onsaid selected best master clock, wherein said best master clock is oneof most stable, most accurate, fastest and slowest.
 10. The apparatusaccording to claim 9, wherein said apparatus is an access point.
 11. Theapparatus according to claim 9, wherein said control logic selects saidbest master clock by executing IEEE 1588 best master clock method. 12.The apparatus according to claim 9, wherein recipients of saidtransmitted message modify a time stamp in said messages responsive totime to receive and process said messages.